Gold filled is a form of plating regulated by the FTC. It is a base metal covered by a layer of gold. If the gold layer is 10K, the gold must equal 1/10 the weight of the metal. If it is 12K or higher, it must equal 1/20. Given that many gold platings can be mere microns thick, gold filled is generally a slightly higher quality plating, but ultimately it is still plating. At some point, the gold WILL wear away, exposing the base metal underneath.

If you are looking for a long-term wear piece, it is generally recommended you buy something solid, so the gold and alloy mix is the same all the way through.